Long ribbons of leaves dangle off a central stalk like the naked ribs of an umbrella that has had the fabric torn off.
This is the aptly named umbrella sedge, or Cyperus involucratus. It’s often grown as a garden plant but occasionally seeds from this east-African native escape the yard and grow in the wild. This plant is said to be a close relative of Egyptian papyrus. It generally likes to grow in boggy areas and wetlands.
You can tell this plant from the other triangle-stemmed sedges around because of the many leaves, which grow beneath delicate clusters of flower spikes. But FYI, the leaves are technically bracts, with the actual leaves existing only as tiny nubs around the base of the stem.